“Off-board tools,” such as scan tools and code readers are known in the art and are testing devices that interface with vehicle diagnostic systems to access, display, and/or print vehicle diagnostic information. OBD II (On-Board Diagnostics version II) Scan Tools are one commonly known type of scan tool and are governed by a number of standards, e.g., SAE J1978 Rev. 1988-02 and SAE J1979 Rev. 1997-09. The term “off-board tools” as used herein is primarily used to describe portable diagnostic devices that are temporarily connected to the vehicle; however, the term “off-board tools” includes diagnostic devices wherein a portion of the diagnostic device is permanently connected to the vehicle.
Protocol determination for off-board tools is an automatic (hands off) determination of the communication protocol that the vehicle is using for the OBD II functions. Many vehicles have multiple computer modules making up a vehicle diagnostic system. These computer modules often use different communication protocols. This automatic determination of the communication protocol is specified in a SAE J1978. In section 6.4.1 and 6.4.2 the SAE specifies a procedure for trying four protocols, J1850 PWM, J1850 VPW, ISO, Keyword 200 and CAN, and determining which protocol to use. The SAE also suggests a method for determining the OBD II protocol in J1978 section 6.4.2.
The vehicle diagnostic system includes one or more computer modules, such as, for example, an electronic control unit (ECU), a transmission control unit, and an anti-skid braking system (ABS), which are all in circuit communications. Typically, an access point, such as, a data link connector (DLC) is used to provide access to the vehicle diagnostic system. An off-board tool is connected to the DLC and linked to the vehicle diagnostic system. Upon linking with the vehicle diagnostic system, the off-board tool communicates with the vehicle diagnostic system by transmitting/receiving messages using a communications protocol. Messages (e.g., requests and instructions) are transmitted by the off-board tool to one or more of the computer modules, such as, for example, the ECU. An acknowledgement is transmitted from the computer module (e.g., the ECU) after the message is received. For example, the off-board tool sends a message request (e.g., a request to perform a predetermined function) to the ECU. After receiving the message request, the ECU transmits an acknowledgement message back to the off-board tool. The acknowledgement message acts as a confirmation that the ECU received the message request. The off-board tool enters an error mode if no acknowledgement message is received from the ECU.
As noted above, one of the communications protocols is the CAN protocol. For vehicles that support the CAN protocol, the off-board tool may communicate with the vehicle diagnostic system according to CAN protocol. The messages transmitted between the off-board tool and the vehicle diagnostic system are formatted according to the CAN protocol. Currently, the CAN protocol standards provide that at least certain messages may be formatted to a ID length of either 11-bits or 29-bits. Although the CAN protocol in the vehicle diagnostic systems are designed to acknowledge and interpret both 11-bit and 29-bit ID messages received from an off-board tool, it may be the case that vehicle diagnostic system's running the CAN protocol do not properly acknowledge 11-bit ID messages transmitted by an off-board tool.